The structure of a conventional vacuum smelting reduction furnace is composed of, as shown in FIG. 1, a reduction retort 1, a condenser 2, and a chamber structure 3. The reduction retort 1 is horizontally disposed and supported by supports 4 and 5, wherein the end of the reduction retort 1 at where the condenser 2 is disposed is projected beyond the chamber structure 3, and the other end of the reduction retort 1, which is closed, is placed within the chamber structure 3. The charging of reactant material and the discharging of spent residue is carried out through the end of the reduction retort 1 at where the condenser 2 is disposed. The conventional reduction retort and the manner it is disposed in the reduction furnace are inconvenient for the charging and discharging processes. It is very labor-intensive and requires a lot of time and energy in production. In addition, it also has low fill rate and low unit throughput due to the small volume of one single reduction retort.
Furthermore, in the metal production process by vacuum smelting reduction method, after charging the reduction retort with the reactant material, depending on the type of metal and reducing agent used, the reduction reaction temperature is generally maintained between 1000 and 1200° C. and the pressure is reduced to vacuum for the reduction reaction to take place. The reduction reaction temperature can be attained by heating the reduction retort with fuel or electricity. The conventional reduction retort is of a circular tubular structure. In the reduction process, heat is transferred from the inner wall of the reduction retort to the reactant material that is in direct contact with the wall. As for the reactant material that is not in direct contact with the inner wall of the reduction retort, heat is transferred thereto through radiation or from other reactant material through conduction. Because of the low thermal conductivity of the reactant material, it is apparent that the temperature of reactant material that is in direct contact with the inner wall of the reduction retort would be elevated much faster than the temperature of the reactant material that has to rely on the heat transfer from the reactant material next to them. Hence, it takes very long time for all the reactant material to reach the needed reduction reaction temperature. The reduction time, from the charging of material to the completion of reduction reaction, for metal production process by vacuum smelting reduction method using conventional reduction retort is generally between 6 and 15 hours. In general, the time needed for a retort with larger diameter would be longer, and so the size of the reduction retort is limited accordingly. Obviously, long reduction time, low production efficiency and output, large energy wastage and high cost are the shortcomings of the conventional reduction retort.
Moreover, in the metal production process by vacuum smelting reduction method mentioned above, the internal pressure of the reduction retort is generally required to be less than 100 Pa. The metallic vapor constantly travels from the reactant material to the condenser during the reduction reaction process. Regional pressure may build up if the metallic vapor is trapped and accumulated within the reactant material pile. The build-up of the regional pressure to more than 100 Pa in the region where the reactant material are thickly piled will inhibit further reduction reaction from taking place, affecting the normal course of reduction reaction, and hence resulting in lower production output and wastage of energy.
Furthermore, the operating condition for a reduction retort in a metal production process by vacuum smelting reduction method is very severe, therefore refractory material is needed to make reduction retorts. However, heat-resisting metal that is resistant to higher temperature and suitable for long duration of use is very expensive. Conventionally, the general material used to make reduction retorts is heat resisting nickel-chrome-steel alloy that has a maximum working temperature of 1200° C. under vacuum condition. The strength of the reduction retort made of this material is often enhanced by thickening its wall. However, this type of reduction retort has a short service life. It is easy to sustain damages like oxidation, creep, tear, etc. at high temperature. Therefore, large quantity of heat-resisting metal is needed to make a reduction retort, leading to high smelting cost. Not only is the need for constant turning and changing of such reduction retort very labor-intensive and time-consuming in production, it also leads to heat loss of the reduction furnace. Besides, the reduction time of metal production process by using such reduction retort, which has a maximum working temperature of 1200° C., is significantly longer as compared to the reduction time of the same process carried out at a temperature much higher than 1200° C.